The holographic information storage refers to a technique that utilizes holographic interference recording to store data on a recording medium. To record, interference of two light waves with one another is required. One of the two light waves is a data-carrying light beam referred to as a signal beam. The signal beam is incident on the recording medium in a fixed direction. Another one of the two light waves is a light beam referred to as a reference beam. Different physical conditions, including incident angle, phase, etc., under which the reference beam is incident on the recording medium are changeable. When the reference beam and the signal beam are incident on the recording medium and interfere with one another, the refractivity of the recording medium changes with the interference of the two beams with one another, so that the recording medium forms a phase grating to enable recording of information thereon. To reproduce the recorded information from the recording medium, a reading beam is incident on the recording medium under the same conditions used by the reference beam in recording the information on the recording medium, and a diffraction signal of the reading beam is the original signal beam to form an image on a light detector, such as a charge-coupled device (CCD). Then, the information is reproduced.
In the process of holographic recording, when the reference beam and the signal beam are incident on the recording medium from the same side of the recording medium, the holographic recording is referred to as a transmissive holographic storage; when the two beams are incident on the recording medium from two generally opposite sides of the recording medium, the holographic recording is referred to as a reflective holographic storage; and when the reference beam and the signal beam are incident on the recording medium from two adjacent sides of the recording medium with an angle about 90 degrees contained between the two beams, the holographic recording is referred to as a 90-degree holographic storage.
In addition, the technique of recording holographic information through changing different conditions of the reference beam is referred to as multiplexing storage, such as wavelength multiplexing storage, angular multiplexing storage, peristrephic multiplexing storage, etc. With the multiplexing storage, a large quantity of data may be stacked and recorded on the same one position of the recording medium to allow a very high storage capacity in the holographic information storage. Among these multiplexing storage techniques, the angular multiplexing storage is one of the most frequently used techniques. With the angular multiplexing storage, each page of data will be recorded using the reference beam at a different incident angle, and the data is stored on the recording medium at the same one position. In other words, the reference beam must be changeable in its incident angle and must always be incident on the same position on the recording medium. As shown in FIG. 11, to fulfill this requirement, an angle-changeable scan mirror la is conventionally used along with a lens assembly 2a to configure a light path for the reference beam. Please refer to FIG. 12. In the prior art, an angle-changeable scan mirror 1b is used to change the direction of the reference beam 2b, and a lens assembly 3b is used to change only the incident angle of the reference beam 2b without changing the incident position of the reference beam 2b on a recording medium 4b, so as to achieve the requirement of angular multiplexing storage. However, the conventional angular multiplexing storage is limited by two things: (1) the aperture size of the optical elements in the lens assembly form a limit to the angle range within which the incident angle of the reference beam may be changed; and (2) the changeability of the optical system in the holographic information recording apparatus forms a limit to the applicability of the holographic information recording apparatus to the above-mentioned three different holographic storage techniques, because it is uneasy to change the optical system once it has been configured.
More particularly, the above-mentioned first limit will lower the system storage capacity. When the limit to the storage capacity by a geometrical configuration is taken into consideration, it can be observed that the storage capacity of the recording medium at the same one position is equal to the total range of changeable incident angles for the reference beam divided by the angular space between recording two adjacent pages. However, the minimum angular space is decided according to the Bragg condition. Therefore, the maximum storage capacity at the same one position on the recording medium is limited by the changeable total recording angle allowed for the reference beam, and the total recording angle is limited by the aperture size of the lenses in the above-mentioned lens assembly in the light path of the reference beam. Therefore, when the above-mentioned prior art is applied in the holographic information storage, the changeable angle allowed for the reference beam will limit the size of the storage capacity. For example, a storage material Model Number HDS2000 supplied by InPhase Technologies is used to estimate the limit to the storage capacity by the aperture size of the optical elements. The value “M/#” corresponds to the sum of the square root of the diffraction efficiency (D.E.) of each recorded page (M/#=Σ√{square root over ( )}D.E.). M/# may be used to assess the total response value of the recording medium. The higher the total response value is, the higher information quantity may be stored on the recording medium. The storage material Model Number HDS2000 has an M/# value of 10 and a thickness of 1 mm. From the Bragg condition, it is estimated the minimum angular space is 0.1 degree, and numerical aperture of the lenses used in the light path for the reference beam is 0.5. And, to obtain better reference beam quality, only the light beam passed through the central area of the lenses is used, and the total recording angle thereof is about 20 degrees. With these conditions, when viewing from the point of geometrical configuration, the holographic storage capacity will be limited to within 200 pages when the angular multiplexing storage technique is used to record. However, when viewing from the point of recording medium quality, given that the diffraction efficiency of data on each page that can be detected by the light detector is 1×10−4, then the storage capacity is 1000 pages. This storage capacity value has exceeded the limit by the geometrical configuration. Therefore, in the event it is desired to fully utilize the storage capacity of the recording medium, the aperture size of the lenses in the light path for the reference beam must be increased. By doing this, it will inevitably increase the volume and weight of the optical elements in the light path for the reference beam and thereby results in a very large and impractical holographic storage system.
The above-mentioned second limit will lower the system flexibility. Since different recording media are different in their characteristics and separately suitable for use with different types of storage configurations, it is necessary to setup different holographic systems for different storage configurations to record holographic information on the different recording media. In the conventional holographic storage systems, there are included a plurality of optical elements. To change the system configuration, the light path must be re-calibrated, and it is uneasy to do so. Particularly, for the angular multiplexing, it involves in very complicated procedures to adjust the optical system of the reference beam. Thus, the conventional optical system consisting of the scan mirror and the lens assembly is not common for the transmissive, the reflective, and the 90-degree holographic storage configuration.
In view of the above two limiting factors in the conventional angular multiplexing storage, it is tried by the inventor to develop an improved holographic information recording and reproducing apparatus to overcome such limits, so that the reference beam is arranged in a new way to allow a versatile information access system and largely increase the storage capacity that is otherwise limited by the geometrical configuration.